Conventionally, tandem color image forming apparatuses in which different color toner images are formed by linearly arranged plural image forming sections, and the color toner images are transferred one by one onto an intermediate transfer medium, which is fed along the image forming sections, to form a combined color toner image thereon, have been used for producing multiple color images at a high speed. In each of the image forming sections of such tandem color image forming apparatuses, an electrostatic latent image formed on a photoreceptor serving as an image bearing member is developed by a developing device using a color toner to form a color toner image on the photoreceptor. The different color toner images thus formed on the photoreceptors of the image forming sections are transferred one by one onto an intermediate transfer medium so as to be overlaid to form a combined color toner image on the intermediate transfer medium, and the combined color toner image is secondarily transferred onto a recording medium such as paper sheets, resulting in formation of a multi-color image.
In order to remove residual toner remaining on the intermediate transfer medium even after the combined color toner image is transferred, a cleaner to remove residual toner using an electrostatic force has been proposed.
For example, JP-2009-258541-A proposes a cleaner which electrostatically removes normally-charged residual toner, which is present on an intermediate transfer medium and which is charged normally (i.e., which has a charge with the same polarity as that of the toner used), and reversely-charged residual toner, which is also present on an intermediate transfer medium and which is charged reversely.
This cleaner includes a first cleaning brush roller to remove normally-charged residual toner from the intermediate transfer medium, and a second cleaning brush roller to remove reversely-charged residual toner from the intermediate transfer medium. In addition, the cleaner includes a first counter roller, which is grounded and which is arranged so as to be contacted with the backside of the intermediate transfer medium while facing the first cleaning brush roller with the intermediate transfer medium therebetween, and a second counter roller, which is grounded and which is arranged so as to be contacted with the backside of the intermediate transfer medium while facing the second cleaning brush roller with the intermediate transfer medium therebetween.
A voltage with a polarity opposite to that of normally-charged toner is applied to the first cleaning brush roller by a power source. In this case, a potential difference is formed between the first cleaning brush roller and the first counter roller, thereby forming an electric field such that normally-charged residual toner on the intermediate transfer medium are electrostatically attracted by the first cleaning brush roller. In addition, a voltage having the same polarity as that of normally-charged toner is applied to the second cleaning brush roller by a power source. In this case, a potential difference is formed between the second cleaning brush roller and the second counter roller, thereby forming an electric field such that reversely-charged residual toner on the intermediate transfer medium are electrostatically attracted by the second cleaning brush roller.
Thus, normally-charged residual toner is electrostatically attracted by the first cleaning brush roller, and therefore the residual toner is removed from the intermediate transfer medium. In addition, reversely-charged residual toner is electrostatically attracted by the second cleaning brush roller, and therefore the residual toner is also removed from the intermediate transfer medium.
In image forming apparatuses, a test pattern of a toner (hereinafter referred to as a toner test pattern) is typically formed on the intermediate transfer medium to improve image quality.
Specifically, when images having a low image area proportion are continuously formed, the amount of toner particles (hereinafter aged toner particles) staying in the developing device for a long period of time increases. Since such aged toner particles have a deteriorated charge property, the quality of images produced by the developing device deteriorates. In order to prevent occurrence of such a problem, a toner test pattern is formed, at a predetermined time, on a non-image area of the intermediate transfer medium to use (remove) such aged toner particles staying in the developing device for a long period of time, followed by supply of a new toner to the developing device to control the toner concentration of the developer in the developing device. This operation is called as a toner refreshing operation. By performing such a toner refreshing operation, the image quality is improved. The toner test pattern formed on the intermediate transfer medium is removed therefrom by a cleaner similarly to residual toner.
In this regard, the amount of toner particles of residual toner, which remains on an intermediate transfer medium even after the secondary transfer process is performed, is about 0.05 mg/cm2 or less. In contrast, the amount of toner particles constituting a toner test pattern is about 1.0 mg/cm2. Thus, the cleaner has to have a function of removing toner particles in an amount of from 0.05 to 1.0 mg/cm2. In this regard, the cleaning current suitable for removing toner particles changes depending on the amount of the toner particles. Specifically, when the amount of toner particles to be removed is large, it is preferable to apply a relatively large current. In contrast, when the amount of toner particles is small, it is preferable to apply a relatively small current.
Therefore, the voltage applied to a cleaning brush roller is typically changed depending on whether toner particles to be removed are residual toner or a toner test pattern, so that the toner particles on the intermediate transfer medium can be satisfactorily removed therefrom. Specifically, when residual toner, which remains on the intermediate transfer medium even after the secondary transfer process is performed, is removed, a relatively low voltage is applied to the cleaning brush roller. In contrast, when a toner test pattern is removed, the voltage is switched to a relatively high voltage.
In addition, the voltage applied to the cleaning brush roller is adjusted depending on the conditions of use. Specifically, the resistances of the intermediate transfer medium and the cleaning brush roller are typically predetermined. However, the resistances tend to change due to variation of initial resistances of such members or when the members are used for a long period of time. When the resistance of the intermediate transfer medium or the cleaning brush roller falls out of the predetermined range and cleaning is performed under the normal conditions, it is possible that defective cleaning is caused.
The cleaning performance of a cleaning brush roller strongly correlates with the amount of the current flowing through the contact portion of the cleaning brush roller and the intermediate transfer medium. If the amount of the current can be maintained so as to fall in the targeted range, it is possible to maintain a high level of cleaning performance even when the resistances of the intermediate transfer medium and the cleaning brush roller change.
There is a technique in that the amount of the current flowing through the contact portion of a cleaning brush roller and an intermediate transfer medium is detected, and the setup voltage value stored in a memory is properly changed so that a targeted current flows through the contact portion of the cleaning roller and the intermediate transfer medium. By using this technique, it is considered that occurrence of defective cleaning can be prevented, because a voltage suitable for cleaning is applied to the cleaning brush roller even when the resistances of the intermediate transfer medium and the cleaning brush roller change.
However, as a result of investigation of the present inventor, it is found that when changing the voltage level in two or more levels, whether or not the voltage can be set to an optimum voltage depends on the method of changing the setup voltage.